The androgen receptor (AR) belongs to the superfamily of steroid/thyroid hormone nuclear receptors, whose other members include the estrogen receptor, the progesterone receptor, the glucocorticoid receptor, and the mineralocorticoid receptor. The AR is expressed in numerous tissues of the body and is the receptor through which the physiological as well as the pathophysiological effects of androgens, such as testosterone (T) and dihydrotestosterone (DHT), are mediated. Structurally, the AR is composed of three functional domains: the ligand binding domain (LBD), the DNA-binding domain, and amino-terminal domain. A compound that binds to the AR and mimics the effects of an endogenous AR ligand is referred to as an AR agonist, whereas a compound that inhibits the effects of an endogenous AR ligand is termed an AR antagonist.
Androgen ligand binding to the AR induces a ligand/receptor complex, which, after translocation into the nucleus of the cell, binds to regulatory DNA sequences (referred to as androgen response elements) within the promoter or enhancer regions of the target genes present in the nucleus. Other proteins termed cofactors are next recruited, which bind to the receptor leading to gene transcription.
Androgen therapy has been to treat a variety of male disorders such as reproductive disorders and primary or secondary male hypogonadism. Moreover, a number of natural or synthetic AR agonists have been investigated for the treatment of musculoskeletal disorders, such as bone disease, hematopoietic disorders, neuromuscular disease, rheumatological disease, wasting disease, and for hormone replacement therapy (HRT), such as female androgen deficiency. In addition, AR antagonists, such as flutamide and bicalutamide, are used to treat prostate cancer. It would therefore be useful to have available compounds that can activate (“agonize”) the function of the AR in a tissue-selective manner that would produce the desired osteo- and myoanabolic effects of androgens without the negative androgenic properties, such as virilization and repression of high density lipoprotein cholesterol (HDL).
The beneficial effects of androgens on bone in postmenopausal osteoporosis were documented in recent studies using combined testosterone and estrogen administration [Hofbauer, et al., Eur. J. Edocrinol. 140: 271–286 (1999)]. In a large 2-year, double-blind comparison study, oral conjugated estrogen (CEE) and methyltestosterone combinations were demonstrated to be effective in promoting accrual of bone mass in the spine and hip, while conjugated estrogen therapy alone prevented bone loss [J. Reprod. Med., 44: 1012–1020 (1999)].
Additionally, there is evidence that hot flushes decrease in women treated with CEE and methyltestosterone; however, 30% of the treated women suffered from significant increases in acne and facial hair, a complication of all current androgen pharmacotherapies [Watts, et al., Obstet. Gynecol., 85: 529–537 (1995)]. It was also found that the addition of methyltestosterone to CEE decreased HDL levels, as seen in other studies. Thus, the virilizing potential and effects on lipid profile of current androgen therapies provide a rationale for developing tissue-selective androgen receptor agonists.
Androgens play an important role in bone metabolism in men [Anderson, et al., “Androgen supplementation in eugonadal men with osteoporosis—effects of six months of treatment on bone mineral density and cardiovascular risk factors,” Bone, 18: 171–177 (1996)]. Even in eugonadal men with osteoporosis, the therapeutic response to testosterone treatment reveals that androgens exert important osteoanabolic effects. Mean lumbar BMD increased from 0.799 gm/cm2 to 0.839 g/cm2, in 5 to 6 months in response to 250 mg of testosterone ester administered intramuscularly. SARMs can thus be used to treat osteoporosis in men.
Androgen deficiency occurs in men with stage D prostate cancer (metastatic) who undergo androgen deprivation therapy (ADT). Endocrine orchiectomy is achieved by long acting GnRH agonists, while androgen receptor blockade is implemented with AR antagonists. In response to hormonal deprivation, these men suffered from hot flushes, significant bone loss, weakness, and fatigue. In a pilot study of men with stage D prostate cancer, osteopenia (50% vs. 38%) and osteoporosis (38% vs. 25%) were more common in men who had undergone ADT for greater than one year than the patients who did not undergo ADT [Wei, et al., Urology, 54: 607–611 (1999)]. Lumbar spine BMD was significantly lower in men who had undergone ADT. Thus tissue selective AR antagonists in the prostate that lack antagonistic action in bone and muscle can be useful agents for the treatment of prostate cancer, either alone or as an adjunct to traditional ADT [See also A. Stoch, et al., J. Clin. Endocrin. Metab., 86: 2787–2791 (2001)].
Tissue-selective AR antagonists can also treat polycystic ovarian syndrome in postmenopausal women. See C. A. Eagleson, et al., “Polycystic ovarian syndrome: evidence that flutamide restores sensitivity of the gonadotropin-releasing hormone pulse generator to inhibition by estradiol and progesterone,” J. Clin. Endocrinol. Metab., 85: 4047–4052 (2000).
SARMs can also treat certain hematopoietic disorders as androgens stimulate renal hypertrophy and erythropoietin (EPO) production. Prior to the introduction of recombinant human EPO, androgens were employed to treat anemia caused by chronic renal failure. In addition, androgens increase serum EPO levels in anemic patients with non-severe aplastic anemia and myelodysplastic syndromes. Treatment for anemia will require selective action such as can be provided by SARMs.
SARMs can also have clinical value as an adjunct to the treatment of obesity. This approach to lowering body fat is supported by published observations that androgen administration reduced subcutaneous and visceral fat in obese patients [J. C. Lovejoy, et al., “Oral anabolic steroid treatment, but not parenteral androgen treatment, decreases abdominal fat in obese, older men,” Int. J. Obesity, 19: 614–624 (1995)], [J. C. Lovejoy, et al., “Exogenous Androgens Influence Body Composition and Regional Body Fat Distribution in Obese Postmenopausal Women—A Clinical Research Center Study,” J. Clin. Endocrinol. Metab., 81: 2198–2203 (1996)]. Therefore, SARMs devoid of unwanted androgenic effects can be beneficial in the treatment of obesity.
Androgen receptor agonists can also have therapeutic value against metabolic syndrome (insulin resistance syndrome, syndrome X), particularly in men. Low levels of total and free testosterone and sex hormone-binding globulin (SHBG) in men have been associated with type 2 diabetes, visceral obesity, insulin resistance (hyperinsulinemia, dyslipidemia) and metabolic syndrome. D. Laaksonen, et al., Diabetes Care, 27 (5): 1036–1041 (2004); see also D. Laaksonen, et al. Euro. J Endocrin, 149: 601–608 (2003); P. Márin, et al. Int. J. Obesity, 16: 991–997 (1992), and P. Márin, et al. Obesity Res., 1(4): 245–251 (1993).
Androgen receptor agonists can also have therapeutic value against neurodegenerative diseases such as Alzheimer's disease (AD). The ability of androgens to induce neuroprotection through the androgen receptor was reported by J. Hammond, et al., “Testosterone-mediated neuroprotection through the androgen receptor in human primary neurons,” J. Neurochem., 77: 1319–1326 (2001). Gouras et al. reported that testosterone reduces secretion of Alzheimer's β-amyloid peptides and can therefore be used in the treatment of AD [(Proc. Nat. Acad. Sci., 97: 1202–1205 (2000)]. A mechanism via inhibition of hyperphosphorylation of proteins implicated in the progression AD has also been described [S. Papasozomenos, “Testosterone prevents the heat shock-induced over activation of glycogen synthase kinase-3β but not of cyclin-dependent kinase 5 and c-Jun NH2-terminal kinase and concomitantly abolishes hyperphosphorylation of τ: Implications for Alzheimer's disease,” Proc. Nat. Acad. Sci., 99: 1140–1145 (2002)].
Androgen receptor agonists can also have a beneficial effect on muscle tone and strength. Recent studies have demonstrated that “physiologic androgen replacement in healthy, hypogonadal men is associated with significant gains in fat-free mass, muscle size and maximal voluntary strength,” [S. Bhasin, et al., J. Endocrin., 170: 27–38 (2001)].
Androgen receptor modulators can be useful in treating decreased libido in both men and women. Androgen deficiency in men is related to diminished libido. S. Howell et al., Br. J. Cancer, 82: 158–161. Low androgen levels contribute to the decline in sexual interest in many women during their later reproductive years. S. Davis, J. Clin. Endocrinol. Metab., 84: 1886–1891 (1999). In one study, circulating free testosterone was positively correlated with sexual desire. Id. In another study, women with primary or secondary adrenal insufficiency were provided physiological DHEA replacement (50 mg/day). Compared with women taking placebo, DHEA-administered women showed an increase in the frequency of sexual thoughts, interest, and satisfaction. W. Arlt, et al., N Engl. J. Med. 341:1013–1020 (1999), see also, K. Miller, J. Clin. Endocrinol. Metab., 86: 2395–2401 (2001).
Additionally, androgen receptor modulators may also be useful in treating cognitive impairment. In a recent study, high-dose oral estrogen either alone or in combination with high-dose oral methyltestosterone was given to postmenopausal women for a four-month period. Cognitive tests were administered before and after the four-month hormone treatment. The investigation found that women receiving a combination of estrogen (1.25 mg) and methyltestosterone (2.50 mg) maintained a steady level of performance on the Building Memory task, but the women receiving estrogen (1.25 mg) alone exhibited decreased performance. A. Wisniewski, Horm. Res. 58:150–155 (2002).